The metal foam material considered is disclosed in WO 2004089564 and WO 2005037467 further disclosing a manufacturing process, according to which the metal foam is produced in sheets with the desired length and width.
Foam based filters are known for diesel particulate filtering applications, although most of them are based on ceramic materials. Typically ceramic foams tend to exhibit pore sizes which need to be sufficiently small to achieve good filtration efficiencies resulting in a relatively high pressure drop, due to the large number of closed pores. Moreover, as more soot is accumulated in the filter, the latter foams are known to lower their filtration efficiency until a “blow-off” behavior, i.e. a negative efficiency, is observed.
Another drawback of the ceramic foams is the limitation in the shaping of the final filter. Therefore, it is very difficult to obtain a large filtration area in the limited space required in automotive applications.
Unlike most commercialized diesel particulate filters, which work based on the surface filtration mechanism, foam filters operate based on deep-bed filtration. This means that the soot is collected within the foam structure rather than on its surface. Based on current experience based on engine testing, a highly efficient filter with small pores will tend to accumulate much more soot near its entrance rather than towards its exit. This non uniform soot distribution is not favorable since the highly soot loaded regions of the filter will result in a non proportional increase of the pressure drop.
In the known prior art, the material proposed is merely referred to generally as porous metal thereby lacking specific teachings on the production process.
Besides, some known embodiments are defined by the need to support the thermal regeneration of the filters via the usage of electrical heaters. Therefore the usage of at least two cylindrical filter elements is called for. Due to the limitations related to the material, the production process of the filter elements and the need of multiple filter elements, the thickness of the filter elements is restricted in the region 0.5 to 20 mm, preferably 1 mm or more.
The pore size range proposed to be used between 100 and 600 μm is fairly small. Similarly the filtration thicknesses are very narrow. Besides, a simple reference is made to a “3-D trapping effect” thereby lacking to assess an understanding of the actual filtration mechanisms.
In EP 879939 of Sumitomo Electric Industries discloses an exhaust gas purifier, with filter elements formed of porous metal. This system needs a heating device for regeneration of the filter. In addition the pore size has to be identical on all filter elements for three reasons. First it makes it easier to manufacture the porous metal filter elements, secondly it avoids concentration of pressure loss on the smaller pores, and thirdly it makes it possible to increase the PM trapping quantity.
The material proposed is generally referred to as porous metal, without specifically detailing the production process. Besides the embodiments of FIG. 1 to 3 are defined by the need to support the thermal regeneration of the filters through the use of electrical heaters, notably at least two cylindrical filter elements. Due to the limitations related with the material, the production process of the filter elements and of the need of multiple filter elements, the thickness of the filter elements is restricted in the region 0.5 to 20 mm and more preferably 1 mm or more.
A still further known prior art DE 4012719 A1 of ROGGENKAMP et al. relates to dynamic foams resulting in the use of ceramic foams resulting in that they are not able to provide a porosity gradient. It includes also a heating device, aimed to burn the accumulated soot quite often, in order to avoid a too high pressure drop. Another drawback of the ceramic foams is the limitation regarding the shaping of the final filter. Therefore, it is very difficult to obtain a large filtration area in the limited space required in automotive applications.
EP 0 603 392 discloses a particulate trap, with filter elements made of mesh-like porous metal, using heat resistant metal. This sort of filter takes advantage of the conductivity of the metal, in order to withstand the temperature changes produced by the filter regeneration.
The pore size range proposed to be used particularly preferably from 100 to 600 μm is fairly small. Similarly the filtration thicknesses are very narrow.